1. Field of the Invention
The present invention relates to a raw material feeding device for a film-forming apparatus for chemical vapor-phase deposition, and more particularly, to a raw material vaporization apparatus and a raw material feeding method for the film-forming apparatus, in which a thin oxide film is formed using a gas sublimated from a metalorganic compound as a raw material.
2. Description of the Related Art
There are known methods for forming a thin film made of ferroelectric oxide such as a sputtering method, a liquid phase epitaxial method, a sol-gel method and the like. As one of such methods of forming a thin oxide film, there is also a metalorganic chemical vapor-phase deposition (MOCVD) in which a high deposition rate and a step-coverage property are obtained and further an epitaxial growth is achieved.
In case that an oxide thin film of e.g., nonlinear optical materials is formed in the MOCVD apparatus, dipivaloyl methane (DPM) complexes are used for solid raw materials since such complexes can be sublimated with a comparatively high vapor pressure at a relatively high temperature, for example, 150 to 200.degree. C.
FIG. 1 is a cross sectional view of a conventional raw material feeding device used in MOCVD apparatus. In the figure, the reference numeral 1 designates a container for a solid raw material, numeral 2 represents a bulk quantity of the solid raw material powder, numeral 3 represents a pair of mesh metal wires for supporting the solid raw material therebetween, numeral 4 represents an inlet conduit for a carrier gas, numeral 5 represents a flange, numeral 6 represents an outlet conduit for a mixture of the carrier gas and the sublimated metalorganic gas connected to a growth chamber, and numeral 7 represents an oven for heating the container.
As shown in FIG. 1, a pair of mesh stainless steel wires 3 hold the bulk solid raw material 2 and are placed in the raw material container 1. The container is heated by the oven 7 to a temperature at which the raw material can vaporize but does not decompose while a carrier gas such as argon (Ar) is introduced through the inlet conduit 4 into the container. The carrier gas passing through the solid raw material 2 carries the metalorganic gas sublimated from the solid raw material 2 as a gas mixture through the outlet conduit 6 and feeds the gas mixture into the growth chamber of the MOCVD apparatus.
In this way, the bulk of the solid raw material powder supported by the mesh metal wires is sublimated while the sublimated metalorganic gas is mixed with the carrier gas directly passing through the bulk material, whereby the gas mixture with the raw material is obtained for the MOCVD.
FIG. 2 shows an initial state of the bulk solid raw material powder interposed between the mesh metal wires 3 in the conventional raw material feeding device, and FIG. 3 shows a state of that bulk after a time of passage of the carrier gas therethrough. In the figures, the reference numeral 2 designates a solid raw material. In FIG. 2, arrow represent the carrier gas uniformly passing through the bulk 2 of the solid raw material powder at the beginning of feeding the gas mixture. In FIG. 3, numeral 21 represents holes, each of the holes penetrating the bulk 2 of the solid raw material powder and arrows in the figures indicate paths of the gas.
In the conventional device for feeding metalorganic gas, the gas mixture including the metalorganic gas at a predetermined density is maintained at a stable rate during the initial state of the bulk of the solid raw material powder under predetermined conditions, since the carrier gas is uniformly infiltrated into the fresh bulk 2 of the solid raw material powder and passes therethrough at the beginning, as seen from FIG. 2. However, after the bulk of the solid raw material powder is repeatedly used to pass the carrier gases through the bulk time after time, the carrier gas forms a plurality of holes 21 which penetrate the bulk 2 to facilitate passage of the gas as shown in FIG. 3. In this case, the sublimation of the material tends to occur mainly in internal surfaces of the holes 21 to consume the bulk of the solid raw material. This phenomenon causes uneven distributions of the passages of the carrier gas.
Furthermore, in addition to the above matter, the solid raw material generally progressively increases in grain size due to re-crystallization caused by the repeated heatings. This phenomenon remarkably promotes such uneven distributions of passages of the carrier gas with the elapse of time.
Therefore, the density of the raw material components in the gas mixture is lower than that of the initial state when the initial conditions for the gas mixture is maintained, so that the predetermined density of the metalorganic gas in the gas mixture becomes unstable with the lapse of time. As a result, the required thin film to be formed is thinner than expectation, so that the thin film becomes rough or uneven.
To make a thicker concentration of the supply of raw material, a measurement was made, in which the raw material held in the container is increased in quantity, or a plurality of raw material containers of the same type are connected together.
In the case where a powder solid raw material is held by wire meshes, it is not possible to obtain a thick raw material gas when the raw material is small in quantity. Also, the solid raw material decreases with the elapse of time during the use of the material, and becomes porous as compared with the bulk material in the early stage. Thus, the area of the material in contact with the carrier gas is varied so that the concentration of obtained raw material in the gas decreases gradually under a constant vaporization condition. Therefore, it is difficult to maintain a constant concentration in the supplied raw material gas.